Information may be more rapidly processed and transmitted using optical as opposed to electrical signals. Optical signals can be used to enhance the performance of electronics processors. For example, electronic wires interconnecting integrated circuits (ICs) can be replaced with optical interconnects and the information processed with IC driven electro-optic modulators. Optical signals in fiber optic communications can be encoded on the optical carrier using electro-optic (EO) modulators. In both of the processes above, nonlinear optical materials with second-order nonlinear optical activity are necessary to affect the modulation of light signal.
Nonlinear optical materials can also be used for frequency conversion of laser light. Such a conversion is desirable in many applications. For example, optical memory media are presently read using 830 nm light from diode lasers. The 830 nm light wavelength limits the spot sizes which can be read and hence the density of data stored on the optical memory media. Similarly, in fiber optic communications, light wavelengths of 1.3 .mu.m or 1.5 .mu.m are desirable due to the low transmission losses of glass fiber at those wavelengths. However, those wavelengths are too long for detection by Si based detectors. It is desirable to frequency double the 1.3 .mu.m or 1.5 .mu.m wavelengths to 650 nm or 750 nm wavelengths where Si based detectors could be used.
Nonlinear optical materials which have been used in electro-optic devices have in general been inorganic single crystals such as lithium niobate (LiNb0.sub.3) or potassium dihydrogen phosphate (KDP). More recently, nonlinear optical materials based on organic molecules, and in particular polar aromatic organic molecules have been developed.
The nonlinear optical properties of organic and polymeric materials has been the subject of numerous symposia. The International Society for Optical Engineering (SPIE) has sponsored a number of NLO related symposia, e.g. the symposium "Nonlinear Optical Properties of Organic Materials II" on Aug. 10-11, 1989 (SPIE Proceedings Series, Vol. 1147, 1990). Similarly, the Materials Research Society has sponsored a symposium titled, "Nonlinear Optical Properties of Polymers" on Dec. 1-3, 1987 (Materials Research Society Symposium Proceedings, Vol. 109, 1988).
The organic based materials have a number of potential advantages over the inorganic and semiconductor based materials. First, the organic materials have higher NLO activity on a molecular basis. Organic crystals of 2-methyl-4-nitroaniline have been shown to have a higher nonlinear optical activity than that of LiNb0.sub.3 Second, the nonlinear optical activity of the organic materials is related to the polarization of the electronic states of the organic molecules, offering the potential of very fast switching times in EO devices. The time response of the system to a light field is on the order of 10 to 100 femtoseconds. In contrast, a large fraction of the polarizability in the inorganic crystals is due to nuclear motions of the ions in the crystal lattice, slowing the time-response of the materials. In addition, the low dielectric constant of the organic materials (e.g. 2-5 Debye at 1 MHz) compared to the inorganic materials (e.g. 30 Debye at 1 MHz) enables higher EO modulator frequencies to be achieved for a given power consumption. Third, the organic materials can be easily fabricated into integrated device structures when used in polymer form.
EP 218,938 and U.S. Pat. No. 4,859,876 have used an approach of incorporating NLO active molecules into amorphous polymer host matrices for NLO media. The NLO molecules are incorporated into the host by blending. Such doped polymers have the advantages of being easily fabricated into thin films suitable for integrated optical devices. The media contain organic molecules (dopants) with nonlinear optical activity with the advantages discussed above. These films must be oriented to achieve a non-centrosymmetric alignment of the NLO chromophores. Such alignment is usually achieved by the application of an electric field across the film thickness while the temperature of the polymeric blend is near its glass transition temperature (Tg). The polymer is then cooled with the field on to lock the oriented molecules in place. EP 218,938 discloses a number of polymer host materials, including epoxies, and many types of molecules which have NLO activity including azo dyes such as Disperse Red 1. It is known that an NLO active material such as azo dye Disperse Red 1, (4,-[N-ethyl-N-(2-hydroxyethyl]amino-4-nitro azobenzene), may be incorporated into a host by simply blending the azo dye in a thermoplastic material such as poly(methylmethacrylate), as described in Applied Physics Letters 49(5), 4 (1986) and U.S. Pat. No. 4,859,876.
While the doped polymer approach offers some advantages over organic and inorganic crystals, the approach has a number of problems. First, the stability of the NLO activity over time of such materials has been shown to be poor. A problem associated with a polymer with NLO properties produced by simply blending NLO molecules into a host polymer is that these polymer materials lack orientational stability. There is significant molecular relaxation or reorientation within a short period of time resulting in a loss of NLO properties. For example, as reported by Hampsch et al., Macromolecules 1988, 21, 528-350, the NLO activity of a polymer with NLO molecules blended therein decreases dramatically over a period of days at room temperature.
U.S. Pat. No. 4,792,208 discloses an article containing an NLO medium which employs various sulfonyl moieties as electron acceptor moieties in polar aligned noncentrosymmetric molecular dipoles. U.S. Pat. Nos. 4,869,847 and 4,859,876 disclose the use of polycarbonate resins as the host material for blended NLO compositions. The use of polycarbonate as a matrix for dye aggregates is disclosed by Wang, U.S. Pat. No. 4,692,636.
In addition, the NLO dopants in the blending polymeric media plasticize the polymer host matrix, lowering the polymer glass transition temperature (Tg). Lowering the polymer Tg has the effect of lowering the temperature stability of the electrically oriented NLO material or NLO medium. Near the Tg, segments of the polymer become mobile and the NLO active dopant molecules which were oriented electrically undergo orientational relaxation. Once orientational relaxation has occurred, the NLO medium exhibits no NLO activity.
A third problem with the doped polymers is the poor solubility of the NLO chromophore in the host matrix. Finally, the NLO chromophores tend to aggregate at relatively low doping levels (e.g. 5-20 percent w/v). Such aggregates scatter light and reduce the transparency of the waveguides to unacceptable levels.
Another disadvantage is that the polymer employed may have a low glass transition temperature, lack sufficient tensile strength, or other desirable properties for optical devices.
There is a continuing effort to develop new nonlinear optical polymers with increased nonlinear optical susceptibilities and enhanced stability of nonlinear optical effects. It would be highly desirable to have organic polymeric materials, particularly polymeric materials based on polycarbonate and polyestercarbonate resins, with larger second and third order nonlinear optical properties than presently used organic electrooptic materials.
The present invention solves the problems identified above with doped polymers, while maintaining the advantages listed for the doped polymers and organic based NLO materials.
It is an object of this invention to make optically transparent polymers incorporating organic molecular structures which exhibit NLO activity upon orientation. It is an additional object of this invention that the polymers comprising the NLO medium have a relatively high glass transition temperature. A high glass transition temperature will correlate with high temperature stability of the NLO medium. The incorporation of the NLO active structures into the polymer backbone has a number of advantages. High levels of NLO chromophore functionalization can be achieved without increasing the scattering losses of waveguides fabricated from the polymer. The addition of the groups which add to the NLO activity of the polymer do not plasticize the polymer and lower the polymer Tg. In fact, such modifications can raise the polymer Tg. That the NLO chromophore is inherent to the polymer backbone increase the orientational stability of the NLO chromophores in the fabricated NLO waveguides, reducing the temporal decay of the NLO activity with time.